netdev - Re: [PATCH net] sctp: not allow to set rto

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Date:   Fri, 25 May 2018 15:13:19 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        davem@...emloft.net, David Ahern <dsa@...ulusnetworks.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        syzkaller@...glegroups.com
Subject: Re: [PATCH net] sctp: not allow to set rto_min with a value below
 200 msecs

On Sat, May 26, 2018 at 01:41:02AM +0800, Xin Long wrote:
> syzbot reported a rcu_sched self-detected stall on CPU which is caused
> by too small value set on rto_min with SCTP_RTOINFO sockopt. With this
> value, hb_timer will get stuck there, as in its timer handler it starts
> this timer again with this value, then goes to the timer handler again.
> 
> This problem is there since very beginning, and thanks to Eric for the
> reproducer shared from a syzbot mail.
> 
> This patch fixes it by not allowing to set rto_min with a value below
> 200 msecs, which is based on TCP's, by either setsockopt or sysctl.
> 
> Reported-by: syzbot+3dcd59a1f907245f891f@...kaller.appspotmail.com
> Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> Signed-off-by: Xin Long <lucien.xin@...il.com>
> ---
>  include/net/sctp/constants.h |  1 +
>  net/sctp/socket.c            | 10 +++++++---
>  net/sctp/sysctl.c            |  3 ++-
>  3 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/sctp/constants.h b/include/net/sctp/constants.h
> index 20ff237..2ee7a7b 100644
> --- a/include/net/sctp/constants.h
> +++ b/include/net/sctp/constants.h
> @@ -277,6 +277,7 @@ enum { SCTP_MAX_GABS = 16 };
>  #define SCTP_RTO_INITIAL	(3 * 1000)
>  #define SCTP_RTO_MIN		(1 * 1000)
>  #define SCTP_RTO_MAX		(60 * 1000)
> +#define SCTP_RTO_HARD_MIN	200
>  
>  #define SCTP_RTO_ALPHA          3   /* 1/8 when converted to right shifts. */
>  #define SCTP_RTO_BETA           2   /* 1/4 when converted to right shifts. */
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index ae7e7c6..6ef12c7 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -3029,7 +3029,8 @@ static int sctp_setsockopt_nodelay(struct sock *sk, char __user *optval,
>   * be changed.
>   *
>   */
> -static int sctp_setsockopt_rtoinfo(struct sock *sk, char __user *optval, unsigned int optlen)
> +static int sctp_setsockopt_rtoinfo(struct sock *sk, char __user *optval,
> +				   unsigned int optlen)
>  {
>  	struct sctp_rtoinfo rtoinfo;
>  	struct sctp_association *asoc;
> @@ -3056,10 +3057,13 @@ static int sctp_setsockopt_rtoinfo(struct sock *sk, char __user *optval, unsigne
>  	else
>  		rto_max = asoc ? asoc->rto_max : sp->rtoinfo.srto_max;
>  
> -	if (rto_min)
> +	if (rto_min) {
> +		if (rto_min < SCTP_RTO_HARD_MIN)
> +			return -EINVAL;
>  		rto_min = asoc ? msecs_to_jiffies(rto_min) : rto_min;
> -	else
> +	} else {
>  		rto_min = asoc ? asoc->rto_min : sp->rtoinfo.srto_min;
> +	}
>  
>  	if (rto_min > rto_max)
>  		return -EINVAL;
> diff --git a/net/sctp/sysctl.c b/net/sctp/sysctl.c
> index 33ca5b7..7ec854a 100644
> --- a/net/sctp/sysctl.c
> +++ b/net/sctp/sysctl.c
> @@ -52,6 +52,7 @@ static int rto_alpha_min = 0;
>  static int rto_beta_min = 0;
>  static int rto_alpha_max = 1000;
>  static int rto_beta_max = 1000;
> +static int rto_hard_min = SCTP_RTO_HARD_MIN;
>  
>  static unsigned long max_autoclose_min = 0;
>  static unsigned long max_autoclose_max =
> @@ -116,7 +117,7 @@ static struct ctl_table sctp_net_table[] = {
>  		.maxlen		= sizeof(unsigned int),
>  		.mode		= 0644,
>  		.proc_handler	= proc_sctp_do_rto_min,
> -		.extra1         = &one,
> +		.extra1         = &rto_hard_min,
>  		.extra2         = &init_net.sctp.rto_max
>  	},
>  	{
> -- 
> 2.1.0
> 
> --
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> 
Patch looks fine, you probably want to note this hard minimum in man(7) sctp as
well

Acked-by: Neil Horman <nhorman@...driver.com>